For the cheeses I have used a Vacherin Fribourgeoi, Campo de Montalban, and an Abbaye de Belloc, however you don’t need these specific ones to make a great open-face sandwich. My German grandmother used Velveeta, and covered the sliced bread with a ridiculous amount of butter.
2-3 types of tasty cheese, sliced
several slices of really good bread
quince or apple jelly, tomato chutney, or mustard
Heat the broiler and toast one side of the sliced bread. Lightly toast the other side and cover it with 2-3 slices of cheese. Run it under the broiler until melted and heated through.
The addition is the best. Depending upon what you have in your refrigerator like a quince and apple jelly, tomato chutney or great tasting mustard, smear a bit on the top and serve the open-face sandwich with pickles.